Natural killer (NK) cells provide significant host defense against tumors and viral and intracellular infections, including Toxoplasma gondii, and Mycobacterium avium-intracellulare, that are important pathogens in opportunistic infections in AIDS patients. Our long-term objective is to determine the molecular basis for NK cell recognition of their targets. Our previous studies are consistent with the hypothesis that the Ly-49 homodimer, expressed by a subpopulation of NK cells, directly engages the alpha1/alpha2 domains of the H-2Dd molecule on the target cell. This results in a global inability of the Ly-49+ NK cell to lyse targets by natural killing and by other stimuli. This in vitro effect is corroborated by in vivo effects of this interaction and by studies demonstrating a physical interaction between Ly-49 and Dd. These properties of Ly-49 on NK cells resemble those of the MHC class-I restricted T cell receptor (TCR) rather than CD8, suggesting that Ly-49 should be belong to a family of polymorphic molecules and that Ly-NK cells should express these Ly-49 related molecules. We have confirmed these hypotheses and have cloned cDNAs encoding molecules highly related to Ly-49. Therefore, NK cells may express multiple inhibitory receptors that interact with different MHC class I antigens, suggesting that the specificity of an NK cell may be determined by its expressed repertoire of Ly-49 related molecules. Thus, we propose the following Specific Aims: 1) Complete cDNA cloning of Ly-49-related molecules. We will isolated C57BL/6-derived cDNAs for additional Ly-49-related molecules predicted by previously reported cDNAs that may represent allelic forms and partial C57BL/6 genomic clones. 2) Examine the expression of Ly-49 related molecules. We will produced monoclonal antibodies specific for the new Ly-49-related molecules. We will determine the distribution of these molecules on NK cells, if these molecules can be simultaneously expressed by a single NK cell, and whether the molecules on NK cells, if these molecules can be simultaneously expressed by a single NK cell, and whether the molecules can form heterodimers with each other. 3) Determine the specificity and function of the Ly-49-related molecules. We will modify our approaches that successfully determined the specificity and function of Ly-49 to efficiently perform these experiments. Thus, these studies should provide significant insight into the molecular basis for NK cell specificity and should delineate the central role of MHC class I in influencing natural immunity. These studies should ultimately lead to the precise identification of the role of NK cells in normal and diseased immune responses and provide potential therapeutic avenues for modulating NK cell activity.